The Primary Aim is to estimate the impact of a 3- and 6-week intake of once daily 325 mg aspirin on the composition of the gut microbiome using a randomized placebo-controlled double-blinded design, i.e. examine the change of microbiome over time within and between the subjects. The hypothesis for this aim is that in the aspirin arm versus the placebo arm, gut microbiome composition will shift towards a lower proportion of pro-inflammatory, CRC-predisposing bacteria (e.g. Fusobacteria) and higher proportion of anti-inflammatory, CRC-protective bacteria (e.g. butyrate-producing bacteria).